The present invention relates to computer controlled industrial machines and, more particularly, to a method for interrupting a machine in any phase of its primary operation, performing a secondary operation and returning the machine to the exact location of its primary operation at which it was interrupted.
Industrial machines, which include machine tools such as milling machines and lathes as well as robot devices such as welding machines and material handling machines, are typically controlled by computer numerical control (CNC) systems. These CNC systems incorporate memory units which are programmed to contain a set of instructions defining a precise machining operation to be performed by a machine. In the case of a lathe, the instruction set will define the movement of a cutting tool with respect to a workpiece so as to enable the lathe to machine the workpiece to predetermined dimensions. In the case of a welding robot, the instruction set will similarly define the movement of a robot arm so as to bring the welding head into proper position and then to effect the welding operation.
In the performance of a machining operation it often becomes necessary to interrupt the operation, manually move the controlled tool or arm and subsequently re-initiate the machining operation at the point of interruption or at an earlier point. For example, a typical machining cycle on a lathe might include cutting threads on a workpiece. If a portion of a threadcutting tool were to break during the threadcutting cycle, the threads would not be properly formed. When the broken tool is detected, an operator will interrupt the cycle, manually move the tool holding fixture to a convenient location, change the tool and re-initiate the machining cycle. If the broken tool was not discovered early, i.e., if some threads had been partially formed using the broken tool, it may be necessary to cause at least a portion of some preceding cutting cycles to be repeated. The system must therefore not only have capability for returning to the point at which the machining operation was interrupted but also must be capable of recalling and repeating an earlier portion of the operation.
In the Mark Century.RTM. 1050 CNC available from General Electric Company, there is provided a tool inspection procedure which allows a machine operator to move a tool away from a workpiece with an automatic return to the point at which machining was interrupted. This CNC system also includes a retrace feature which permits an operator to have the CNC repeat a limited number of machining steps occurring prior to the point of interruption. However, the tool inspection procedure can only be implemented when machining has been stopped and the retrace function is limited to programmed linear moves. Accordingly, the inspection and retrace features cannot be used in a threadcutting mode. Furthermore, since the tool inspection procedure requires that the tool first be stopped, if such were attempted in a threadcutting mode, the workpiece would be irrepairably damaged by "ringing" of the thread.
It is an object of the present invention to provide an improved method for interruption of a machining operation with recovery to the exact point of interruption.
It is another object of the invention to provide an improved method for interruption of a machining operation which can be implemented without initial stoppage of the machining operation.